Atrial fibrillation is the most common arrhythmia and accounts for significant morbidity and mortality. Recently, evidence has suggested that atrial fibrillation may be a result of focal activity from the atrial myocardium of the pulmonary veins. However, the mechanism of this form of atrial fibrillation is not known nor is the reason why there is a predominance of these foci in the pulmonary veins. It is the long-term goal of this application to study the electrophysiologic properties of the atrial myocardium of the pulmonary veins and define structure/function relationships of the arrhythmogenic substrate. In particular, we will utilize high resolution optical mapping of cardiac activation in a canine model of atrial dilatation and atrial fibrillation to identify the precise mechanism of focal atrial fibrillation. In the first Specific Aim, we will identify the potential substrate for arrhythmias in the myocardium of the pulmonary vein and identify the role of triggers such as calcium channel agonists and a-adrenergic agonists. In the second Specific Aim, we will study the role of acute pulmonary vein dilatation on the substrate and triggering of focal atrial fibrillation. In the third Specific Aim, we will study the effect chronic pulmonary vein dilatation on the substrate and mechanism of focal atrial fibrillation. In the last Specific Aim, we will study how focal discharges from the pulmonary vein myocardium interact with the vulnerable substrate of the remainder of the atrium to produce sustained atrial fibrillation.